Charged particle beam systems, which are devices where an electron is brought to a final focus using magnetic or electrostatic lenses, have their performance limited at this time by fourth and higher order aberrations of the lenses. Examples of such electron beam devices are electron microscopes, electron microanalyzers, ion microscopes, and scanning transmission electron microscopes.
A system for correcting spherical or third order aberration of lenses in electron beam devices was shown and described in U.S. patent application Ser. No. 088,224, an invention of Albert V. Crewe and David A. Kopf, entitled "Sextupole System for the Correction of Spherical Aberration", filed Oct. 25, 1979 U.S. Pat. No. 4,303,864. In that application, a sextupole focusing arrangement was described for compensating for spherical aberration in charged particle beam devices. In that arrangement, a sextupole was positioned between two focusing lenses. The third order aberrations of a sextupole are cylindrically symmetric and of opposite sign to those of the upstream focusing lens. By producing a beam crossover in the center of the sextupole, its aberrations were used to correct the third-order aberration of the upstream round lens. However, no workable solution for eliminating higher than third order aberration in charged particle beam systems was known. These higher order aberrations limit the usefulness of charged particle beam devices in that probe sizes cannot be reduced below a specified minimum, the sizes of acceptable images are larger than desired, and magnification of the images must be limited to an unacceptably low level.
It is therefore an object of this invention to provide a means for compensating for third order, as well as higher than third order aberration in a charged particle beam system.
Another object of this invention is to provide a means for compensating for higher than third order aberration in charged particle beam systems having as few elements as possible, preferably no more than three elements.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.